It is very necessary to monitor the state of blood oxygen for patients in the process of operation and reablement, generally, by monitoring a parameter of blood oxygen saturation. Conventionally, the above parameter is measured with spectrophotometry which utilizes the difference between light absorption coefficients of reduced hemoglobin and oxyhemoglobin based on the Lambert-Beer law and the theory of light scattering. The spectrophotometer can be performed by transmitted light or reflected light. The Lambert-Beer law is expressed as:I=I0e−∝d,Where I is the intensity of transmitted light, I0 is the intensity of incident light, C is the concentration of the light-receiving matter in solution, d is the path length of light absorbed by solution, and ε is the light absorption coefficient of the matter. From the above equation, the absorbance D is reached as follows:D=ln I0/I=εcd. It indicates that the light absorption of the matter correlates with the concentration thereof which implies the possibility of calculating internal composition of tissues from the light absorption of them.
The researchers have further researched the reduced hemoglobin (Hb) and the oxyhemoglobin (HbO2) closely correlating with the blood oxygen saturation. It is found that the difference between the light absorption coefficients of HbO2 and Hb is notable, as shown in FIG. 2, in which the solid line represents the light absorption coefficient-wavelength curve of HbO2, and the dotted line represents the light absorption coefficient-wavelength curve of Hb. It is shown in FIG. 2 that the light absorption coefficient of HbO2 is only one tenth ( 1/10) of that of Hb for the visible red light with wavelength of 660 nm, but the light absorption coefficient of HbO2 is greater than that of Hb for the infrared light with wavelength of 940 nm, and the light absorption coefficients of HbO2 and Hb have one isoabsorption point for the bred light with wavelength of 805 nm.
The arterial blood oxygen saturation is defined as:SaO2=HbO2/(Hb+HbO2)=C1/(C1+C2),  (1)where C1 is the concentration of HbO2, and C2 is the concentration of Hb. SinceD(660)=ln I0(660)=ln(I0(660)/I(660)e−ε1c1de−ε2c2d)=ε1c1d+ε2c2d,  (2)D(805)=ln I0(805)/I(805)=ln(I0(805)/I(805)e−ε3c1de−ε4c2d)=ε3c1d+ε4c2d,  (3)where ε1 and ε2 are the light absorption coefficients of HbO2 and Hb for the red light with wavelength of 660 nm respectively, ε3 and ε4 are the light absorption coefficients of HbO2 and Hb for the infrared light with wavelength of 805 nm respectively and both equal to ε (i.e. ε3=ε4=ε), and d is the thickness of the light-transmitting tissue, the following equations can be reached:C1+C2=D(805)/εd, C1=(D(660)−ε2D(805)/ε)/(ε1−ε2)d. By substituting them into the equitation (1), the following equation is reachedSaO2=A×D(660)/D(805)+B,  (4)where A=ε/(ε1−ε2) and B=ε2/(ε1−ε2).
However, D(660) and D(805) are not only relevant to Hb and HbO2, as expressed in the equations (2) and (3), but also relevant to the absorption of muscles, bones, pigments, adiposes, venous blood and the like in tissues. That is, each of D(660) and D(805) should further include a portion of background absorption as shown in FIG. 3, so the equations (2) and (3) becomeD(660)=ln I0(660)/I(660)=ln(I0(660)/IBe−ε1c1Δde−ε2c2Δd)  (5)D(805)=ln I0(805)/I(805)=ln(I0(805)/IBe−ε3c1Δde−ε2c2Δd)  (6)where I0 is the intensity of incident light, IB is the intensity of transmitted light when only the background absorption of tissues presents, Δd is the variation of the transmission distance as a result of the change from blood-free to blood-perfused. The background absorbance is easily defined as:DB=ln(I0/IB).Thereby, the following equations can be reached:D(660)−DB(660)=ε1C1Δd+ε2C2Δd,  (7)D(805)−DB(805)=ε3C1Δd+ε4C2Δd,  (8)where ε3=ε4=ε, so the equation (4) becomeSaO2=A×(D(660)−DB(660))/(D(805)−DB(805))+B.  (9)The equation (9) is the fundamental formula for detecting the blood oxygen saturation.
Generally, the infrared light with one isoabsorption point for wavelength of 805 nm is not utilized to detect the blood oxygen saturation, because it is hard to acquire the precise value of such wavelength and resultantly relatively large error occurs. The infrared light with wavelength of about 940 nm is commonly utilized, for the reason that the variation of the light absorption coefficients of HbO2 and Hb for the wavelength around are more smooth and thus little error usually occurs. When the infrared light with wavelength of 940 nm is utilized, since ε3 is not equal to ε4 (i.e. ε3≠ε4) in the equation (8) the equation (9) becomes the blood oxygen saturation Spo2 Spo2=(A×R+B)/(C×R+D),  (10)where A=ε1, B=−ε2, C=ε4−ε3, D=ε1−ε2, and
                    R        =                                                            D                ⁡                                  (                  660                  )                                            -                                                D                                      B                    ⁢                                                                                                                ⁡                                  (                  660                  )                                                                                    D                ⁡                                  (                  940                  )                                            -                                                D                  B                                ⁡                                  (                  940                  )                                                              .                                    (        11        )            It can be known from the above equations that “R” and blood oxygen saturation are one to one correspondence. Since D=LnI0/I=εcd,
                              R          =                                                                      ln                  ⁢                                                                          ⁢                                                            I                                              R                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      RM                                                                      -                                  ln                  ⁢                                                                          ⁢                                                            I                                              R                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      Rm                                                                                                                    ln                  ⁢                                                                          ⁢                                                            I                                              I                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      IM                                                                      -                                  ln                  ⁢                                                                          ⁢                                                            I                                              I                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      Im                                                                                            =                                          ln                ⁢                                                                  ⁢                                                      I                                          R                      ⁢                                                                                          ⁢                      m                                                        /                                      I                    RM                                                                              ln                ⁢                                                                  ⁢                                                      I                    Im                                    /                                      I                    IM                                                                                      ,                            (        12        )            where IRM is the maximum intensity of the transmitted light of red light, IRm is the minimum intensity of the transmitted light of red light, IR0 is the intensity of the incident light of red light, IIM is the maximum intensity of the transmitted light of infrared light, IIm is the minimum intensity of the transmitted light of infrared light, and II0 is the intensity of the incident light of infrared light. With regard to red light, the following equation can be reached:
                              ln          ⁢                                          ⁢                                    I              Rm                        /                          I              RM                                      =                              ln            ⁡                          (                              1                -                                                                            I                      RM                                        -                                          I                      Rm                                                                            I                    RM                                                              )                                .                                    (        13        )            When the ratio of pulsating component to direct current (DC) component, namely (IRM−IRm)/IRM is small,
      ln    ⁡          (              1        -                                            I              RM                        -                          I                              R                ⁢                                  m                  ≈                                                                          I            RM                              )        ⁢                    I        RM            -              I                  R          ⁢                      m            ≈                                      I      RM      pulsating component/DC component.Accordingly, R can be expressed as follows:
                    R        =                                            Red              AC                        /                          Red              DC                                                          Ir              AC                        /                          Ir              DC                                                          (        14        )            where RedAC is the alternating current (AC) component of the intensity of transmitted red light (i.e. AC peak value of the intensity of red light), RedDC is the DC component of the intensity of transmitted red light, IrAC is the AC component of the intensity of transmitted infrared light (i.e. AC peak value of the intensity of the infrared light), and IrDC is the DC component of the intensity of transmitted infrared light. From the above equations, it can be seen that the main factor influencing the variable R is the AC components of the intensity of transmitted red light and infrared light, because the DC components of the intensity of the two transmitted lights are relatively stable for a period of time after the operating state of the light emitting diode is adjusted and fixed. Now the AC component is calculated by finding out the maximum value and minimum value of the intensity of the two transmitted lights. Therefore, the value of “R” can be calculated if the waveforms of the two transmitted lights in a full pulse wave were known.
In a human body, the arterial blood pulsates in the end parts of tissues as a result of the pulse wave, and the HbO2 and Hb cause the end parts of tissues (such as fingers) to have different transmittivities for red light and infrared light. Nowadays, according to the above principle, the domestic or foreign pulse oximeters operate by irradiating red light and infrared light with a certain intensity to the fingers, detecting the transmitted light intensities of the two lights, and then calculating the blood oxygen saturation based on the ratio of the density variations of the red light and the infrared light after the two lights passing through the fingers and the corresponding equations described above.
According to the principle described above, a device for measuring blood oxygen saturation basically includes a blood oxygen sensor and a signal processing unit. The key element of the blood oxygen sensor is a sensor including a light-emitting diode (LED) and a photosensor. The LED can provide the lights of two or more wavelengths. The photosensor can convert the light signals passing through the fingers and containing the information of blood oxygen saturation into electrical signals which are provided to a signal processing module to be digitalized for calculating the blood oxygen saturation.
More particularly, the measuring device can be functional divided into the following parts, i.e. a power supply circuit, a driving circuit a signal amplifying and processing part, an A/D (analog/digital) converting circuit, a logical control part, a single-chip microcomputer data processing part and the like. Specifically, as shown in FIG. 1, the measuring device includes: a power supply circuit which outputs two groups of power supply to the whole measuring device, wherein one is +5V for digital circuit and the other one is ±5V for analog circuit, while an AC or DC power supply of ±12V is input; a driving circuit adjusted by the logical control part to output currents with different amplitudes for driving the LED, in order to ensure that a light-receiving device (for example photocell) can output signals with certain amplitudes; a sensor part detecting the light signals passing through the fingers and then converting the light signals into electrical signals which are transmitted to a signal amplifying and processing part; the signal amplifying and processing part which applies differential amplifying process, background photocurrent cancel process, gain adjusting and bias current cancel process to the electrical signals and transmits the electrical signals so-processed to an A/D converting circuit the A/D converting circuit converting the electrical signals to digital signals which is transmitted to a single-chip microcomputer to be processed; the single-chip microcomputer data processing part whose calculation module simulates and analyzes the waveform based on the sampled signals to find out the maximum value and minimum value of pulse waveform and then calculates the peaks value of the pulse waveform and the blood oxygen saturation; a serial port circuit through which the parameters of pulse wave described above and the blood oxygen saturations isolated by optical coupler are transmitted, wherein the logical control part is utilized to make various parts under the controls of the single-chip microcomputer, such as the control over light-emitting sequential of the sensor, the control over driving current, the control over bias current, the control over background light cancel, the control of signals A/D converting and the like.
However, there are the following disadvantages in the conventional method described above. The level of perfusion is usually very low for patients. Since it is necessary to measure the AC component of the pulse waveform under this condition, that is, to find out the maximum value and minimum value of the waveform, but the signal to be measured is very poor under low perfusion and the signal-to-noise ratio (SNR) is very low as well, it becomes difficult to find out the waveform. Therefore, errors may occur during the process of measuring the peak value of the pulse wave, and the ratio of AC to DC obtained thus may be wrong, which cause the value of blood oxygen content measured finally to have very low accuracy.